Defective interfering (DI) genomes have a truncated version of the full-length genome and are replicated and packaged into virions by the self-replicating virus from which they originated, or by another genetically compatible, replication competent virus. Until recently understanding of DI viruses and their genomes was based on data obtained from virus-infected cells in the laboratory. Now, in a significant breakthrough, DI genomes similar in sequence and structure to those found in the laboratory have been identified in samples taken directly from human infections caused by a number of different viruses, suggesting that their viromes include DI viruses. As the presence of DI genomes in patients did not appear to correlate, directly or inversely, with the severity of disease the clinical significance of this finding is unclear at the moment, but their detection suggests that the presence of DI viruses is a natural phenomenon in infections in vivo. More work is needed to establish the nature of the role that DI genomes play within the virome.